So I'm reading a book set in the American South in the beginning of the 1900 and I stumble upon the use of the verb is with you ("you is", "is you?") in conversations: eg. "is you Samson Fuller?". I've heard this in rap songs before, but this usage made me wonder how it came about. Could someone give any insight into this? History, usage today?

(The writer also adds an s to some verbs, eg. "I doubts that", "I sees", etc. What's with that?)

"You is," and "Is you," are typically used in literature and in life by many people from the Southern States of the United States. The primary usage dates back to the times when so many of the people from this area were practically illiterate and/or had little education as a byproduct of being heavily farming regions with little use for proper schooling. As most of the foreman and farmhands were, well, farmers, and not educators, the language was passed along to the slaves in their charge, thus leading to the misuse of the verb to be (an many others), "I seen 'em first," etc. As writers took on the task of writing about these times, so did they take on and pass along the same form of the language.